Wave guide terminating device



J1me 1956 F. L. HENNESSEY WAVE GUIDE TERMINATING DEVICE Filed March 26,1953 TOR FRANK L. H ENNESSEY ATTORNEY-s United States Patent WAVE GUIDETERMINATING DEVICE Frankv L. Hennessey, Washingtoml). C., assignor tothe United States of America as represented by the Secrelaw of. the NavyApplication March 26, 1953', SerialNo. 344,915

6 Claims. (Cl. 343-781) (Granted under Title35, U. S. Code (1952), see.266) This invention relates to broad band feed systems for microwaveantennas utilizing beam concentration reflecting surfaces, and moreparticularly to terminations for such feedsystems.

More specifically this invention concerns itself with the provision of avertex feed system for microwave antenna systems that utilize afocal-fed-parabolic reflecting surface. This type of antenna feed systemgenerally comprisesa straight section of waveguide projecting'throughthe vertex of the reflector and terminated at the focus of the reflectorby a radiating'element that directs the energy back to the reflector.Such a system is shown and described in the test Principles of Radar, byReintijis and C'oate (McGraw-Hili), pp. 956957, wherein arectangular'waveguide is shown terminated in a so-called' Cutler feed.

When it is desired to rotate the beam produced by such an antenna in anarrow cone in space by rotating the focal endof thewaveguide in a smallcircle about the focus, such as shown in Patent No. 2,617,029 to Plummetet al., the use of a waveguide of circular cross-section in a vertexfeed system is peculiarly advantageous. The circular waveguide may bereadily rotated while mainraining a fixed orientation of polarization sothat it is only necessary to cock the guide slightly off' the axis. ofthe reflector near the focus to achieve the desired conical beam. Thefeed systems that have been developed inthe past are generallyrestricted to use at a givenv frequency or at best over a narrow band offrequencies. Reflections back into the waveguide that produce a largemismatch and loss of power have invariably resulted when the operatingfrequency is only slightly removed from the design frequency.

One object of this invention is to provide-an improved vertex feedsystem for use with afocal-fed reflecting surface wherein a circularwaveguide projects through the vertex of the reflecting surface.

Another object of this invention is to provide such a vertex feed systemsuitable for use over a wide range of frequencies.

A still further object of this invention is to provide a vertexfeedsystem for use with a focal-fed reflector, producing essentially a pointsource of radiation to thereflector.

Other objects and features of the present inventionwill become apparentupon consideration of the following;

illustration only. and not as a definition of the limits of theinvention, reference for the latter purpose being. had

to the appended claims.

Figure l is a perspective view of an antenna system suitable forpracticing the present invention.

" Figures 1a and 1b show side sectional and end elevaducting surface maybe described as being a figure of lice.

2 tion views respectively of one embodimentof the present invention.

Figure 1c shows a partial perspective view of a modification of theembodiment of Figures la and 1b resulting in a more simplifiedconstruction.

Figure 2a shows an exploded view of another embodiment of the presentinvention suitable for use when. a sharper point source is desired.Figure 2b is an end view of this embodiment;

Figure 3 is a perspective view of still another embodi ment of theinvention that is useful where it is desired both to rotate the feed andto decrease the size of the point source of radiation to the paraboloid.

Briefly the antenna feed structure of the present invention comprises aconducting surface of unique efformation disposed at the focal end ofthe feed'waveguide so as to intercept and split the wave emanating fromthe guide, and to redirect it around the terminating edges of, the guideback to the reflector. In more particular the C011? revolution generatedby rotating a semi-circle about an axis which is coaxial with thewaveguides axis. and which is perpendicular to a line connecting theends'of the semicircle at, or very near, one end thereof. Such a figureofrevolution may be further defined as semi-toroidal in shape containingan apex-like protuberance at the axis of revolution. The apex of thegenerated conducting surface is positioned on the longitudinal axis ofthe guide so that the annular concave conducting surface of thesemi-toroid is disposed aboutv the end of the guide and is'facing theantenna reflector. The apex. of the conducting-surface is thus placed ina region where the magnetic flux of electromagnetic radiation oftransverse electric mode of propagation emanating from the guide is veryweak. A linearly polarized wave of TE1 modehas a concentration ofelectric lines of force at the axis of the guide, which lines aredividedby the conducting surface as the waves progress out of. the guide,terrmnatingthereafter on the annular concave face of the semirtoroidalconducting surface and on the edge of the guide untilthe energy islaunched into space toward' the. antenna re.- flector. When theconducting surface isappropriat'ely. spaced from the end of the guide,it has beenfound that the wave propagation through the guide is notdisturbed, and that electromagnetic energy isdirected toward the antennareflector with an impedance match that. will give unexpectedly smallstandingv wave ratios over a Wide variation in operating frequencies.Furthermore it has been discovered that for optimum operation the radiusof the semi-circle of revolution above-described. must be. very nearlyequal to the radius of the waveguide. While satisfactory operation maybe obtained when the ratio of the radius ofthe semi-circleof revolution.to the radius of the guide is other than very nearly unity, there. willbe a decrease in the frequency spectrum over which a. given maximumstanding wave ratio can be obtained.

Additionally, it has been found that a circularlypolarized wave may beredirected toward, the antenna: reflector, with a standing wave ratioequally low asv that obtained. with linearly polarized waves. The reasonfor, this is that orthogonally polarized modes may be readily re-.directed by the feed system because of the axial sym: metry of theconducting surface. Inasmuch as a circu-. larly polarized wave-may beconsidered to be the sum of two orthogonal linearly polarized waves,circular.

of paraboloidal reflector 1 is mission of electromagnetic energy from aconventional external source. The waveguide is terminated at the focusof the paraboloid by the conducting surface provided by the presentinvention and indicated in general at 117.

With reference now to Figure in, there is shown in detail one embodimentof the present invention wherein components corresponding to those ofFigure I bear corresponding identifying numerals. Circular waveguide 101is of three distinct sections, two cylindrical sections 103 and 107being joined by a truncated conical section 105, cylindrical section 103having a larger inner diameter than cylindrical section 107. Fittedwithin waveguide section 107 is a dielectric plug member 109, typicallymade of polystyrene. The outer diameter of this member is the same asthe inner diameter of waveguide section 107 along the length ofwaveguide section 107, but tapers to a point 111 within conical section105. At the end of dielectric member 109 opposite the tapered point 111is an end section 113 of rather critical dimensions. End section 113 isof semi-toroidal shape, the radius of curvature of the surface of thetoroid being approximately half the outer diameter D of the waveguidesection 107. As has been previously indicated, the semi-toroidal surfacemay be considered to have been generated by revolving around the axis ofthe guide a semi-circle of diameter A tangent to said axis so as to forman apex thereat. Fitted snugly over end section 113 is a metal cap 117,of Monel metal or any other metal that is a good electrical conductor.It should be noted that waveguide 101, dielectric member 109 and metalcap 117 are distinct members, and may be readily disassembled each fromthe other.

The outer dimensions of cap 117 are not particularly critical but theinner curved dimensions are quite critical; for optimum results the capmust fit very snugly over dielectric end section 113 and its centralpoint, or apex, 115 must be very exactly positioned on the axis ofwaveguide section 107. The diameter A of the curved sur face of metalcap 117 and dielectric end section 113 (twice the radius of curvature ofthe toroidal surface of end section 113) need not be exactly the same asthe outer diameter of waveguide section 107, but it has been found thatthis relationship gives optimum results. For every dimension of diameterA there is a distance L between the tip of the apex 115 of metal cap 117and the transverse plane of terminating edge 121 of the waveguide thatwill give optimum results as described below. This dimension L" must bedetermined experimentally for every metal cap by axially movingdielectric member 109 within guide 107 until the best impedance match isobtained. Generally, as the ratio of A to D is increased, it has beenfound that the apex 115 should project farther into the waveguide 107.Typically at an operating frequency of 9,375 mc., a distance L of .043inch has been found satisfactory for a cap having a radius of curvatureof .33 inch, and a major or outer diameter B of 1.345 inches, using awaveguide of .685 inch outer diameter and .05 inch wall thickness. Ithas been found that a difference of 7% between A and D, or of .02 inchin the axial positioning of a cap of the dimensions given above isenough to produce a decided adverse eifect on the impedance match.

Using a waveguide and a, metal cap of the dimensions and relativepositioning given above, a standing wave ratio of less than 1.4 wasobtained over a frequency range of about 7800 to about 10,800 mc. Priorart devices could be operated with a standing wave ratio of less than1.4 only over a 3% frequency range, typically of 9150 mc. to 9450 me.

The function of the dielectric member 109 is primarily to provide bothmechanical support for metal cap 117 and a means to axially positionsaid metal cap 117. It

a waveguide 101 for transshould be noted however that another usefulpurpose is served by member 109 in that its presence in the feed guide101 permits a reduction in the permissible diameter of the waveguidesection 107 where the dielectric member is positioned over the diameterof guide section 103 where an air dielectric is employed. Thesignificance of this reduction in diameters is that a point source ofradiation is more nearly achieved at the focus of the reflector. Thetapered section 111 of the dielectric plug 109 and conical waveguidesection 105 are utilized to provide a good impedance match betweenwaveguide sections 103 and 107. Dielectric member 109 including endsection 113 may be eliminated if desired so long as other means areprovided for positioning the metal cap 117 which do not adversely affectthe propagation of the radiation.

The operation of the embodiment of Figure 1 is believed to be asfollows. Assume that a vertically polarized TEu wave is being propagatedin the waveguide. Upon reaching the point of the metal cap 117, electriclines of force of the wave will be progressively divided by the metalcap starting at the tip 115. Effectively, the wave is then transmittedaround terminating edge 121, the electric lines of force beingterminated on edge 121 and on the interior of the toroidal conductingsurface of the metal cap 117. It has been found that the waves aredivided and swung around terminating edge 121 in such a manner as to bealmost entirely redirected toward the paraboloidal reflector.

The metal cap 117 may be eliminated if desired by coating the exteriorsurface of dielectric end section 113 with a metal film. Such a deviceis shown in Figure 10, wherein numeral 123 denotes the metal film orcoating over the end section 113. Care must be taken to insure that theconductive coating goes to the tip of cavity 125 so that the waves willbe properly divided and redirected. The operation of the device is thesame as described above for Figure 1a, conductive coating 123 performingthe function of the concave, interior surface of metal cap 117.

It should be noted that apex 115 need not come to a sharp point, but maybe blunted somewhat. In the specific example given above it will benoted that there is .025 inch difference between the outer diameter B ofthe toroidal surface and the diameter A of the figure of revolution.This difference is the diameter of the blunted apex.

It has been found that if linearly polarized waves are used a pointsource of radiation may be more nearly achieved and the antenna gainconsiderably increased by cutting away part of the end section 113 andmetal cap 117 in the manner shown in Figure 2. For satisfactory results,the cut-away sections have a radius of curvature the same as that of theoutside diameter B of dielectric end section 113, but this dimension isnot critical. The. radii of curvature are centered so as to cut only tothe outer surface of waveguide section 107. For correct operation usingthis feed, it is necessary to polarize the radiation along the majoraxis of the resulting end-section, as shown by polarization indicatingarrows 133. Under this condition the section of the full feed of Figure1a that is cut-away in the cut feed of Figure 2 would have little effecton the incident wave.

The spoked cap embodiment shown in Figure 3 combines the better featuresof the cut feed of Figure 2 with the ability of the full feed of Figurel to transmit radiatron of any polarity. In this embodiment the metalcap 117 comprises a solid base member 129 snug-fitted ever end section113 and having a diameter essentially equal to the outer diameter ofwaveguide section 107. Uniformly radially extending from the base member129 are a number of spokes or fins 131 snug-fitting on the outer portionof end section 113. spokes nearly perpendicular to the E-vector havelittle eifect on this incident wave, only the spokes more nearly It hasbeen found that the parallel to the electric field direction through theaxis of the guide being effective to redirect the radiation.

Although the embodiments disclosed in the preceding specification arepreferred, other modifications Will be apparent to those skilled in theart which do not depart from the scope of the broadest aspect of thepresent invention.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereof or therefor.

What is claimed is:

1. In a vertex feed system for an antenna having a beam concentratingreflector and a section of tubular waveguide projecting through thevertex thereof to the focus thereof; a conductive member disposed at theend of said waveguide, said member having a concave surface facing thereflector, said surface being generated as a figure of revolution formedby revolving a semi-circle about an axis perpendicular to the diameterof said semicircle and coaxial with the waveguide axis thereby to forman apex on said axis; and adjustable positioning means operable toposition the apex of said surface along the axis of said waveguide atthe focal end thereof with said apex pointing toward said beamconcentrating reflector.

2. In a vertex feed system for an antenna having a beam concentratingreflector and a section of tubular waveguide projecting through thevertex thereof to the focus thereof; a conductive member disposed at theend of said Waveguide, said member having a concave surface facing thereflector, said surface being generated as a figure of revolution formedby revolving a semi-circle about an axis perpendicular to the diameterof the semicircle and coaxial with the axis of the waveguide thereby toform an apex at said axis, said conducting member being symmetricallycut away on two opposite sides thereof up to the projection of saidwaveguide on said conducting surface, and adjustable positioning meansoperable to position the apex of said surface along the axis of saidwaveguide at the focal end thereof with said apex pointing toward saidreflector.

3. A termination for a tubular waveguide for redirecting electromagneticradiation around an end thereof comprising a conductive reflectingsurface having a concave reflective surface configuration substantiallythat of a surface of revolution derived by rotating a semi-circle aboutan axis tangential to said semi-circle at a diametric end thereofforming an apex at the said axis, and means operative to position saidreflecting surface at the end of said waveguide symmetrically withrespect to the axis thereof with said apex pointing toward saidWaveguide.

4. A termination for a tubular waveguide for redirecting electromagneticradiation around an end thereof comprising a conductive reflectingsurface having a concave reflective surface configuration substantiallythat of a surface of revolution derived by rotating a semi-circle aboutan axis tangential to said semi-circle at a diametric end thereofforming an apex at the said axis.

5. In a vertex feed system for an antenna having a beam concentratingreflector and a section of tubular Waveguide projecting through thevertex of the reflector to the focus thereof; a conductive memberdisposed at the end of said waveguide, said member having a concavesurface facing the reflector, said surface being generated as a figureof revolution formed by revolving a semicircle having a diametersubstantially equal to the outer diameter of said waveguide about anaxis perpendicular to the diameter of said semi-circle and coaxial withthe waveguide axis thereby to form an apex on said axis; and adjustablepositioning means operable to position the apex of said surface alongthe axis of said waveguide at the focal end thereof with said apexpointing toward said beam concentrating reflector.

6. The feed system described in claim 1 wherein the said conductivemember is a finular member with the fins thereof radially extending fromthe waveguide axis and said fins are individually shaped to collectivelyform the described surface configuration.

References Cited in the file of this patent UNITED STATES PATENTS2,206,923 Southworth July 9, 1940 2,483,575 Cutler Oct. 4, 19492,505,424 Moseley Apr. 25, 1950 2,535,331 Swarts Dec. 26, 1950 2,617,029Plummer et al. Nov. 4, 1952

